1. Field of the Invention
The present invention relates to an optical access network system and a multicast communication method thereof, and more particularly to an OLT (Optical Line Terminal) provided as a center side network element in an optical access network system and a multicast communication method to be used in the optical access network system in which the quality of multicast service is improved.
2. Description of the Related Art
An xDSL (x-Digital Subscriber Line) is the generic term of ADSL (Asymmetric Digital Subscriber Line), HDSL (High-bit-rate DSL), RADSL (Rate-Adaptive DSL), SDSL (Symmetric DSL) and VDSL (Very-high-bit-rate DSL). This xDSL is a modem technology allowing for the high-speed packet communications at several tens Megabits/sec at maximum, using the existing Subscriber Line (ordinary telephone cable made of copper wire) as a transmission line. Due to the introduction of communication services employing the xDSL, a high-speed and always-connected internet access network has become popular and widely spread.
However, as the xDSL technology involves the packet communication using the telephone cable, it has a problem that the transmission characteristics and the data transmission speed are affected by the length of the telephone cable, the characteristics of the telephone cable, and the peripheral environmental conditions of wiring path of the telephone cable from a telephone switching office to the subscriber's premises.
Thus, an access network using the optical technologies has been widely spreading, instead of the access network using the xDSL technology. The access network using the optical technologies is an optical access network so called an EPON (Ethernet Passive Optical Network) which employs the Ethernet technologies and realizes the packet communication through an optical cable connected to the subscriber's premises. The PON technology is recommended in the IEEE (The Institute of Electrical and Electronics Engineers, Inc.) 802.3ah.
The optical access network by the EPON is composed of an OLT (Optical Line Terminal) that is installed in the switching center of a communication common carrier as a center side network element, and an ONU (Optical Network Unit) that is installed in the subscriber's premises as a subscriber side network terminal. This optical access network is constructed by laying one optical fiber cable to an area in which plural subscribers' premises are locating, connecting a splitter as an optical coupler to the optical fiber cable for branching an optical path into a plurality of optical paths, and connecting each of branched optical cables to respective subscriber's premises. The optical access network can provide the user with the packet communications of wider band and higher quality than the access network with the metallic cable such as the telephone cable. Particularly, the optical access network is most suitable for an application such as the moving picture contents distribution services.
In the moving picture contents distribution services, a relatively large amount of data is transmitted in real time to a plurality of users. Therefore, from the viewpoint of efficiently using the network, the multicast services which perform the Point-to-Multipoint communications are suitable for the application such as the image distribution services. The services which have been widely spreading as the multicast services are the services implemented by the IP (Internet Protocol) communications. In an IPv4 (Internet Protocol version 4), a router takes control of whether or not there is any host participating in the multicast services on a subnet, using an IGMP (Internet Group Management Protocol: Version 1 and Version 2). In an IPv6 (Internet Protocol version 6), there is an MLD (Multicast Listener Discovery) that is a multicast group management protocol for use between the router and the host. The IGMP or MLD is the protocol for managing and controlling the IP multicast group, and used by a multicast router supporting the multicast services and the client terminals receiving the multicast data.
In Japanese Patent Laid-Open No. 2004-343243, a protocol for implementing the multicast through the IP communications is disclosed. Herein, the multicast router periodically multicasts a Query message packet representing a processing request as a character string to the terminals. The terminal having received this Query message packet returns a Report message packet to the multicast router. This Report message packet contains the multicast address of the multicast group in which the terminal participates.
The multicast router having received the Report message packet recognizes the terminal which declares a participation intention in the multicast group, and reflects the information of the Report message packet to the routing process. Thereby, the multicast data packet is distributed to a plurality of terminals from the multicast router.
On the other hand, when a terminal wants to leave from the multicast group in which it participates, it transmits a Leave message packet. The multicast router having received the Leave message packet stops distributing the multicast data packet to the terminal declaring the leave intention.
In the above disclosed technology, the multicast service is provided in accordance with the above protocol procedures. Further, the multicast service is realized, without providing the functional blocks of routing on the subscriber side, by snooping (monitoring) each protocol packet when multicasting and performing a logical link control in conjunction with the snooped result.
Also, in Japanese Patent Laid-Open No. 2004-214758, a communication system is proposed in which the multicast data packet is distributed to each terminal, regardless of the number of multicast distribution destinations, without copying it by the number of distribution destinations.
FIG. 8 is a system block diagram representing the essence of the conventional typical optical access network system. An OLT (Optical Line Terminal) 101 as a center side network element in the conventional optical access network system 100 comprises a control board 102 for controlling the entire apparatus, and a plurality of PON interface boards (PON I/F) 1031, 1032, . . . and 103n. This conventional optical access network system 100 with the OLT 101 is composed of a subscriber side network terminal portion and a network side apparatus portion. The subscriber side network terminal portion comprises the optical fiber cables 1041, 1042, . . . and 104n connecting to the respective PON interface boards 1031, 1032, . . . and 103n, the 1×N splitters 1051, 1052, . . . and 105n for splitting the optical fiber cables 1041, 1042, . . . and 104n into n×N optical paths corresponding to subscribers, not shown, and a plurality of ONUs (Optical Network Units), as the subscriber side network terminal, 10611 to 1061N, 10621 to 1062N, . . . and 106n1 to 106nN connected to the respective optical paths branched by 1×N splitters 1051, 1052, . . . and 105n. And the network side apparatus portion is configured such that the SNI (Service Node Interface) ports 1081, 1082, . . . and 108n, which are interface ports to the network side on the PON interface boards 1031, 1032, . . . and 103n of the OLT 101, are connected to an L2 switch 109.
The conventional OLT 101 is configured such that an SNI port 108, which is connected to the L2 switch 109 as a part of the network (not shown), is provided for each PON interface board 103. Accordingly, when the OLT 101 accommodates the PON interface boards 1031, 1032, . . . and 103n, the respective SNI ports 1081, 1082, . . . and 108n are connected to the L2 switch 109, for example. And these n SNI ports 1081, 1082, . . . and 108n assure the connection of respective PON interfaces with the network.
FIG. 9 is a system block diagram representing how to transfer the multicast data in the conventional optical access network system. The same parts are given the same numerals throughout FIGS. 8 and 9, and the explanation of the same parts is omitted properly.
In FIG. 9, a router 121 and the first and second L2 switches form a part of the network to which the optical access network system 100A is to be connected, and a router 121 is connected to the first and second L2 switches 109A and 109B. The first L2 switch 109A is connected via the first OLT 101A and the 1×N splitters 1051A, 1052A, . . . and 105nA to a plurality of ONUs 10611A to 1061NA, 10621A to 1062NA, . . . and 10611A to 106nNA. The circuit configuration of first OLT 101A and each of ONUs 106n1A to 1061NA, 10621A to 1062NA, . . . and 106n1A to 106nNA is the same as that of shown in FIG. 8 respectively. The circuit portion of the second L2 switch 109B, the second OLT 101B and the equipment connected to the second OLT 101B is substantially the same as the circuit portion of the first L2 switch 109A, the first OLT 101A and each of ONUs, and the illustration and explanation of this circuit portion are omitted properly.
In FIG. 9, the router 121 distributes a multicast data packet to both of the first and second L2 switches 109A and 109B. The first L2 switch 109A is representatively explained below.
The first L2 switch 109A copies and distributes the multicast data packet as indicated by line 131 to the respective SNI ports 1081, 1082, . . . and 108n of the first OLT 101A. Each of the PON interface boards 1031, 1032, . . . and 103n distributes the multicast data packet received from the corresponding SNI port 1081, 1082, . . . or 108n to the respective ONUs 10611 to 1061N, 10621 to 1062N, . . . and 106n1 to 106nN as indicated by the lines 1321 to 132n.
With the conventional multicast communication method as described in Japanese Patent Laid-Open No. 2004-343243 or Japanese Patent Laid-Open No. 2004-214758, there is a problem that the multicast data packet has a deleterious effect on the communication band and communication quality for non-multicast data packets in the PON section between the OLT 101 in FIG. 8 and the plurality of ONU 10611 to 1061N, 10621 to 1062N, . . . and 106n1 to 106nN, for example.
This is caused by broadcasting the multicast data packets between the OLT 101 and each of the plurality of ONU 10611 to 1061N, 10621 to 1062N, and 106n1 to 106nN with the conventional technologies as described in Japanese Patent Laid-Open No. 2004-343243 or Japanese Patent Laid-Open No. 2004-214758. That is, all the multicast data packets transmitted by each of the PON interface boards 1031, 1032, . . . and 103n are broadcasted and distributed to all the subscribers in the same PON section. Therefore, the multicast data packets arrive at all the subscriber terminals. Accordingly, the subscriber terminal, not shown, makes a filtering process of taking in only the corresponding data packet addressed to the self-terminal, and discarding other data.
Owing to the features of the PON section, the data packet other than the multicast packet (hereinafter referred to as a unicast data packet) is also broadcasted like the multicast data packet. Therefore, each subscriber terminal discards the data packets not addressed to itself through the filtering process. Then, if a number of subscribers accommodated in the PON interface board 103 of the OLT 101 each participate in different multicast groups, the multicast data distributed to the PON section increases in proportion to the number of multicast groups. As a result, there is a problem that the communication band for non-multicast data packets is oppressed.
A second problem is that the multicast data packets have a deleterious effect on the communication band and communication quality for the unicast data packets in the SNI ports 1081, 1082, . . . and 108n. This is because if a number of subscribers accommodated in the PON interface board 103 of the OLT 101 participate in the different multicast groups, the number of multicast data packets distributed from the L2 switch 109 to the SNI ports 108 proportionally increases, so that the communication band for the unicast data packets is oppressed.
A third problem is that the load on the L2 switch 109, to which the optical access network system 100 is connected, increases, and it causes to impose a limitation or deterioration on the data band or quality of the provided multicast data services.
The reason is as follows. That is, in the conventional OLT 101, the multicast data packets or unicast data packets are processed in non-concentration manner which performs packet processing individually by each of the PON interface boards 1031, 1032, . . . and 103n. Therefore, there is a problem that the L2 switch 109 connected with the plurality of the SNI ports 1081 to 108n in the OLT 101 has to copy and distribute the multicast data packets by the number of SNI ports 1081, 1082, . . . and 108n. As the data band and quality of multicast services depend on the processing performance of the L2 switch 109, an excessive load of the L2 switch 109 causes to impose limitations regarding the multicast service expansion over the optical access network system, or it causes to deteriorate the quality of multicast services.